.Improving Plants by Selection or Breeding. 553 



ordinary herds is accomplished by the introduction of improved blood 

 through the male. In plant-breeding it is desirable that the seed of the 

 select individuals be planted in a held by themselves. This insures that 

 only progeny of carefully selected plants will be planted near together, 

 and thus no ordinary stock will enter in as a contamination. One can 

 be certain that each plant of the progeny is fertilized with pollen from 

 another similarly good plant, or at least from a plant derived from good 

 parentage. One difhculty, however, has been experienced by plant- 

 breeders in the case of plants which normally cross-fertilize, in planting 

 continuously their selected stock in such isolated plots. If this method 

 is continued year after year, it results in fairly close inbreeding, which in 

 the case of plants frequently results in a loss of vitality and vigor. In 

 animal-breeding it is apparently the case that ordinarily with careful 

 selection, there is no noticeable effect from close inbreeding, and many 

 of the most famous animals have been produced as a result of the closest 

 in-and-inbreeding. In plants, however, it is possible to secure much 

 closer inbreeding than in the case of animals, as in many cases a plant 

 can be fertilized with its own pollen. 



Within recent years much activity has been developed in the careful 

 breeding and improvement of corn. The corn plant has been shown, 

 as a result of experiments carried on by various investigators, as, for 

 instance, the Illinois Experiment Station and by the U. S. Department of 

 Agriculture, to lose vitahty very rapidly when self-fertilized. Within 

 three or four generations by careful self-fertilization it is possible to 

 produce a strain of corn of almost total sterility. The general practice 

 of corn-breeders who have been giving attention to the production of 

 highly bred strains is to plant the rows of corn from different select 

 ears side by side, giving a row to each select ear, and each year selecting 

 from the progeny of those rows which give the largest yield, further 

 plants to continue the selection. Planting these select ears together 

 every year, therefore, means that they are more or less inbred as the 

 closest relatives are planted together in the same row. While in follow- 

 ing this policy at first no effect was visible, corn-breeders are now find- 

 ing in some cases an apparent decrease in yield, which seems to be 

 traceable to the effect of inbreeding. It, therefore, seems necessary for 

 us in com and in other plants that are affected by inbreeding, to devise 

 methods that will avoid close inbreeding. The detrimental effect 

 of inbreeding is largely limited to those plants which are normally 

 cross-fertilized, this fact being strikingly brought out in Darwin's exten- 

 sive "Investigation on Cross-and Self-fertilization in the Vegetable King- 

 dom." Tobacco, wheat, and some other plants which are normally 



